The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
When an internal combustion engine is initially started, the engine and exhaust system are cold. Further, the intake manifold absolute pressure (MAP) is near atmospheric pressure because the engine is initially turning at a very low rate. The low temperature and high pressure make it difficult for fuel to vaporize in the cylinders. Incompletely vaporized fuel is not completely combusted. The portion of fuel that is not completely combusted at start-up and the period shortly after start-up creates an exhaust mixture that is fuel-rich. The fuel-rich mixture increases hydrocarbon and carbon monoxide levels in the exhaust.
Vehicles including an internal combustion engine may also include a catalytic converter to treat exhaust. The catalytic converter catalyzes the oxidation of hydrocarbon molecules and carbon monoxide molecules as well as the reduction of nitrogen oxides emitted in the vehicle exhaust. The catalytic converter does not efficiently control emissions at start-up because: 1) a higher fraction of fuel remains unburned and is subsequently exhausted, and 2) the catalyst has not reached an appropriate operating temperature.
The catalytic converter must reach an operating temperature referred to as the light-off temperature to operate efficiently. The catalytic converter reaches light-off temperature after the engine and exhaust system have been adequately heated from engine operation. The period of engine operation required to heat the catalyst provides for sufficient engine RPM to provide a low MAP. The combination of a low MAP and increased operating temperature allows the fuel to vaporize adequately, resulting in more complete combustion of fuel.
Strategies have been employed to reduce exhaust emissions and increase catalytic converter performance at start-up. Such strategies include increasing engine speed at idle, retarding ignition timing, reducing delivered fuel, and adding a secondary air pump to the exhaust stream that supplies the catalyst with oxygen to carry out the oxidation reaction.